docs: def _update_info(self):
          """
          Call parent method and make sure this is in fact a
          table HDU.  Set some convenience data.
          """
          super(TableHDU, self)._update_info()
          if self._info['hdutype'] == IMAGE_HDU:
              mess = "Extension %s is not a Table HDU" % self.ext
              raise ValueError(mess)
          if 'colinfo' in self._info:
              self._colnames = [i['name'] for i in self._info['colinfo']]
              self._colnames_lower = [
                  i['name'].lower() for i in self._info['colinfo']]
              self._ncol = len(self._colnames)